1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a high voltage field effect transistor and a method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for increasing a secondary breakdown voltage.
2. Discussion of the Related Art
Power MOSFETs have a high switching speed comparing to the other power devices. Specifically, high voltage lateral power MOSFETs have been preferred as large-integrated power devices in recent years since an ON resistance is low in a device of less than 300 V having a relatively low internal pressure.
For example, the high voltage power device includes DMOSFET (Double-diffused MOSFET), IGBT (Insulated Gate Bipolar Transistor), EDMOSFET (Extended Drain MOSFET), and LDMOSFET (Lateral Double-Diffused MOSFET).
Among these devices, LDMOSFETs are widely used for HSD (High Side Driver) and LSD (Low Side Driver), or H-bridge circuits. Although the LDMOSFETs are easy to fabricate, a threshold voltage becomes high because a dopant concentration of the channel region, which determines the structure of the LDMOSFET, is not uniform. Thus, a breakdown at the surface of the silicon substrate occurs in a drift region neighboring the channel. Accordingly, the EDMOSFET has been recently developed to overcome the above-mentioned disadvantages.
An EDMOSFET according to a background art will be described wit h reference to the attached drawings.
FIGS. 1 and 2 are cross-sectional views illustrating an n-channel EDMOSFET and a p-channel EDMOSFET according to the background art.
Initially referring to FIG. 1, a structure of the n-channel EDMOSFET according to the background art includes a p-type drift region 2 and an n-type drift region 3 respectively formed in a p-type well 1. The n-type drift region 3 has a lightly doped drain (LDD) region in the vicinity of a gate to form a channel. On the p-type well 1 having the p-type drift region 2 and the n-type drift region 3, a gate electrode 4 is formed to have a gate insulating layer 32 interposed therebetween. One side edge of the gate electrode 4 is positioned at the boundary between the p-type drift region 2 and the n-type drift region 3.
In the p-type drift region 2, a heavily doped n-type source region 3 is formed around one side of the gate electrode 4, while a heavily doped p-type impurity region 6 for an electrical contact is at one side of the source region 5. A heavily doped n-type drain region 7 is spaced apart at a predetermined distance from the gate electrode 4 in the n-type drift region 3.
A source electrode 8 is disposed on the heavily doped n-type source region 5 including the heavily doped p-type impurity region 6. A drain electrode 9 is formed on the heavily doped n-type drain region 7, while a field plate 10 is formed over the one side edge of the gate electrode 4 and the n-type drift region 3.
On the other hand, a p-channel EDMOSFET according to the background art has a similar structure to the n-channel EDMOSFET except for conductivity types. As shown in FIG. 2, an n-type drift region 12 and a p-type drift region 13 are formed in an n-type well 11. The p-type drift region 13 has an LDD in the vicinity of a gate to form a channel. On the n-type well 11 having the n-type drift region 12 and the p-type drift region 13, a gate electrode 14 is formed to have a gate insulating layer 32 interposed therebetween. One side edge of the gate electrode 14 is positioned at the boundary between the n-type drift region 12 and the p-type drift region 13.
In the n-type drift region 12, a heavily doped p-type source region 15 is formed around one side of the gate electrode 14 and a heavily doped n-type impurity region 16 for a body contact is around one side of the source region 15. A heavily doped p-type drain region 17 is spaced apart at a predetermined distance from the gate electrode 14 in the p-type drift region 13.
A source electrode 18 is disposed on the heavily doped p-type source region 15 including the heavily doped n-type impurity region 16. A drain electrode 19 is formed on the heavily doped p-type drain region 17, while a field plate 20 is over the one side edge of the gate electrode 14 and the p-type drift region 13.
The operation of such a EDMOSFET according to the background art will be described as follows.
Since the operation of the n-channel EDMOSFET is the same as that of the p-type channel EDMOSFET, only the former will be described for example.
Upon applying a voltage higher than a threshold voltage to the gate electrode 4 and a voltage higher than the voltage of the source electrode 8 to the drain electrode 9, the voltage flows to the drain region 7 through the n-type drift region 3 through the channel region below the gate electrode 4 from the source region 5. At this stage, the breakdown voltage is increased because the field plate 10 prevents a breakdown at the end portion of the gate neighboring the drain region 8. Furthermore, when an appropriate voltage is applied to the field plate 10, a current path of the drift regions will be adjusted, thereby reducing a conduction resistance.
The MOSFET can be operated by two different methods. One method is to apply a gate voltage to the field plate 10 to raise the breakdown voltage and simultaneously to employ characteristics of the conduction resistance. The other is to reduce the conduction resistance by applying a constant voltage to the field plate spaced apart from the gate electrode.
However, the aforementioned EDMOSFET and LDMOSFET according to the background art have the following problems.
Although the EDMOSFET and LDMOSFET according to the background art is designed to have a primary breakdown voltage `high` without applying a voltage to the gate electrode, a secondary breakdown voltage is decreased when a voltage is applied to the gate electrode. Accordingly, the EDMOSFET or LDMOSFET according to the background art, which is designed to have the primary breakdown voltage as high as about 170 V, has the secondary breakdown voltage not higher than about 60 V when 20 V is applied to the gate electrode.